a) Field of the Invention
The present invention relates to lockup control apparatus and method for a vehicular torque converter with which an automatic transmission is provided, having a lockup mechanism for establishing a direct connection between input and output elements of the torque converter. Specially, the present invention relates to a lockup control for releasing a lockup (state) according to an engine speed while a vehicle is coasting (or, during a coasting state).
b) Description of the Related Art
The torque converter transmits a driving power between input and output elements of the torque converter via a working fluid. Hence, a power transfer efficiency in the torque converter is normally low although a torque fluctuation absorbing function and a torque increasing function are achieved. Therefore, a lockup-type torque converter, which is provided with a lockup clutch for establishing a lockup state where input and output elements of the torque converter are directly connected, is commonly used.
In the vehicle equipped with such a lockup-type torque converter, when the vehicle starts to coast with an accelerator opening angle being completely closed, the lockup state is generally applied so as to mainly improve an energy consumption. The lockup state in this situation is hereinafter called a coast lockup. In this coast lockup state, suppose that a lockup clutch engagement differential-pressure that is applied to the lockup clutch is kept the same high pressure as at the time of an ordinary lockup. In this case, when the vehicle rapidly decelerates and wheels are locked while coasting, a release of the lockup for the lockup clutch tends to be delayed. Hence, there is a possibility that an engine stall may be caused by the locked wheels.
With this fact taken into consideration, a Japanese Patent Application First Publication No. 1999-182672 published on Jul. 6, 1999 exemplifies a previously proposed lockup control apparatus in which the lockup clutch engagement differential-pressure (or, pressure-differential) in the coast lockup state (hereinafter, also called a coast lockup differential-pressure) is set to lower than the lockup clutch engagement differential-pressure of a steady-state lockup applied in (or, under) a steady-state traveling state or a slow accelerating state.
Namely, the lockup clutch engagement differential-pressure in the steady-state traveling state is set to be a high differential-pressure almost near a maximum differential-pressure in order to avoid a slipping of the lockup clutch. Then, after the vehicle begins to coast with the accelerator opening angle (detected by an accelerator pedal) being completely closed (detected by an idling switch), the lockup clutch engagement differential-pressure in the coast lockup state (or, the coast lockup differential-pressure) is controlled to be a sufficiently low differential-pressure within a range where the lockup clutch does not judder.
Moreover, a Japanese Patent Application First Publication No. 1999-159608 published on Jun. 15, 1999 exemplifies another previously proposed lockup control apparatus. The above-described Japanese Patent Application First Publication teaches that a controller determines whether the lockup clutch generates a minute slipping in the coast lockup state, and the coast lockup differential-pressure is so corrected (or, updated) and learned as to become smaller gradually while the minute slipping is not detected. In this previously proposed lockup control apparatus, after the process of the correction and learning proceeds enough, the coast lockup differential-pressure can become smaller close to a limit of a range where the minute slipping is not generated even if there is some disparity in a differential-pressure control system. Hence, the release of the lockup is quickly performed when the vehicle is braked suddenly.